The present invention relates to a photoreceptor for electrophotographic, more particularly to a high-sensitive photoreceptor for electrophotography containing a novel azo compound.
Photoreceptor using inorganic photoconductors such as selenium, cadmium sulfide, zinc oxide and the like have been employed in electrophotography. Recently, however, the photoreceptor using organic photoconductors (OPC), or so-called organic photoreceptors have come to be employed for PPC and printers because of many advantages of OPC such as having no pollution problem, easiness of production and handling, capability of forming high-quality pictures and easy preparation into photoreceptor in various forms such as drum, sheet, belt, etc., and the demand for such organic photoreceptor is increasing every year. Organic photo-receptors have indeed many advantages over the conventional inorganic type photoreceptors but they are still inferior to the conventional photoreceptors in sensitivity and durability and therefore at present mainly used for low-speed devices.
Many attempts have been made for improving the sensitivity and durability of organic photoreceptors.
The first commercially available organic photo-receptor was the one making use of the sensitizing action of a charge-transfer complex comprising a mixture of polyvinyl carbazole (PVK) and 2,4,7-trinitrofluorenone (TNF) which is an electron attractive compound.
This has initiated the development of many charge-transfer complex type organic photoreceptors, but there is yet available none which excels said PVK-TNF type photo-receptors in performance.
The organic photoreceptors which are popularly used in the art at present are the so-called function-separated type in which the function of generating the charge carrier and the function of transporting it are separatively performed by the different compounds.
The function-separated type photoreceptors have the merits that it is possible to combine a compound with high efficiency in generating charge carrier and a compound with high transport efficiency, and also the range of choice is wide for materials with excellent durability, so that there can be obtained a photoreceptor having high sensitivity and excellent durability.
There are two types of function-separated type photoreceptor: layered type in which the carrier generating material and the carrier transport material form separate layers and laminated, and single layer type in which said both materials are contained in a same sensitive layer. In both types, there proceeds in the medium the process of generating a carrier in the carrier generating material as it absorbs light, injection the generated carrier into the carrier transport material and transport the carrier between the molecules of said carrier transport material. Therefore, the sensitivity of the photoreceptor depends on the carrier generation efficiency, injection efficiency and transport efficiency, so that there is selected a combination of a carrier generating material with high carrier generation efficiency and a carrier transport material with high carrier transport efficiency and the combination with high carrier injection efficiency.
Various types of photoconductive pigments have been developed as carrier generating material. Especially azo-pigments have been studied preferentially as carrier generating material for PPC as they have high sensitivity and are also excellent in spectral sensitivity as compared with other types of pigments, and some of such azo-pigments have been used practically.
However, the conventional azo-pigments are still unsatisfactory in durability as they exhibit large light-fatigue.
The object of the present invention is to provide a photoreceptor improved in sensitivity and durability, both of which are unsatisfactory in the conventional organic photoreceptors. It is particularly envisaged in this invention to provide an organic photoreceptor which is high in sensitivity, highly resistant to light-fatigue, minimized in reduction of electrostatic properties after repeated use and excellent in durability.
Concerning azo-pigments used as carrier generating material, bis-azo pigments with "Naphthol AS" type coupling component have been studied and reported in many literatures, for instance, Japanese Patent Application Laid-Open (KOKAI) No. 47-37543. The azo-pigments with this type coupling component, however, have defects that they show large light-fatigue and also poor durability for electrostatic properties.
As a result of extensive researches on finding novel azo compounds usable as a carrier generating material for obtaining a photoreceptor for electrophotography with high sensitivity and durability, the present inventors found that certain specific coupler-asymmetric bis-azo compounds are suited for the purpose and accomplished the present invention based on this finding.